Monica Camacho-Perez came to the United States from Mexico as a child, crossing into Arizona with her mother in the same spot where her father made the trip before them. “Nobody stopped us,’’ Camacho-Perez, now 20, said of her 2002 journey.

Three years ago, her uncle tried to cross the border and join the family in Baltimore, where they remain illegal immigrants. He was stopped three times by the U.S. Border Patrol and jailed for 50 days.

As the Department of Homeland Security continues to pour money into border security, evidence is emerging that illegal immigration flows have fallen to their lowest level in at least two decades. The nation’s population of illegal immigrants, which more than tripled, to 12.2 million, between 1990 and 2007, has dropped by about 1 million, according to demographers at the Pew Research Center.

A key — but largely overlooked — sign of these ebbing flows is the changing makeup of the undocumented population. Until recent years, illegal immigrants tended to be young men streaming across the Southern border in pursuit of work. But demographic data show that the typical illegal immigrant now is much more likely someone who is 35 or older and has lived in the United States for a decade or more.

During a recent immigration discussion, Democratic presidential candidate Hillary Clinton claimed that detention centers privately contracted with the federal government have a legal requirement to fill beds. (C-SPAN)

Homeland security officials in the Obama and George W. Bush administrations — who have more than doubled the Border Patrol’s size and spent billions on drones, sensors and other technology at the border — say enhanced security is driving the new trends.

“We have seen tremendous progress,” said R. Gil Kerlikowske, commissioner of U.S. Customs and Border Protection, which is part of the Department of Homeland Security. “The border is much more secure than in times past.”

The issue of border security is central to the broader debate over immigration reform that has roiled Washington in recent years and is emerging as a flash point in the 2016 presidential campaign. Congressional Republicans have insisted on greater border security before they consider legalizing any immigrants who came to this country without proper documents.

President Obama says the border has never been more secure and is urging a series of legislative steps to legalize millions of undocumented immigrants, streamline the visa system and further fortify the border. He has already moved to protect certain undocumented immigrants from deportation through executive actions. But these actions have faced resistance in the courts, including the decision Tuesday by a federal appeals court to keep one of the president’s signature immigration efforts from moving ahead.

What’s increasingly clear is that the shifting fortunes of the U.S. economy account for less of the ebb and flow of illegal immigration. Even as the economy bounces back from recession, illegal immigration flows, especially from Mexico, have kept declining, ­according to researchers and government data. Since the 1990s, the opposite was true: The better the economy, the more people tried to come.

“Every month or quarter that the economy continues to improve and unauthorized immigration doesn’t pick up supports the theory that border security is a bigger factor, and it’s less about the economy and we have moved into a new era,’’ said Marc Rosenblum, deputy director of the U.S. immigration program at the Migration Policy Institute.

Some researchers say factors other than security are playing a role and might even account for much of the reduced flow of illegal immigrants. These researchers point, for instance, to changes in Latin America that could be pushing fewer people to seek a better life in the United States.

At odds with the government’s claims of success, a series of academic studies in recent years have found little linkage between border security and illegal migration.

Douglas S. Massey, a Princeton University sociologist, said the falling numbers of immigrants have “nothing to do with border enforcement.” Massey, who helps run a project that has interviewed thousands of illegal Mexican migrants over the past three decades, attributed the trend to demographic changes in Mexico, such as women having fewer children.

But even some researchers who are skeptical about the overall effectiveness of enhanced border security acknowledge indirect effects of these measures. For potential migrants who are calculating the pros and cons of trying to cross the border, stiffer U.S. security measures are making the trip much more expensive, in particular the exploding cost of hiring a guide. The journey has also become more arduous and dangerous, in part because the DHS has plugged traditional crossing points and driven migrants deeper into the desert.

‘More difficult and expensive’

Since the Bush administration, the DHS has dramatically increased its efforts to lock down the southwest border. The budget for Customs and Border Protection has grown to $10.7 billion in the past decade, a 75 percent increase. The number of Border Patrol agents at the border has nearly doubled over the past decade, to more than 18,000 today.

Much of the ramp-up occurred during the Bush administration, but the Obama administration has marshaled more forces as well. Homeland Security Secretary Jeh Johnson has recently set up three task forces to increase coordination within the DHS.

Current and former DHS officials acknowledge that a confluence of factors explains the decline in illegal migration, including demographic changes in Mexico, improvements in its economy and Mexico’s crackdown on Central American migrants headed to the United States. But these officials insist that the massive investment to secure the border has been the key factor.

“It used to be that you could literally sit at a bar in Tijuana, Mexico, look across the border into San Diego, wait for the Border Patrol to drive in the other direction and make a run for it,’’ said Steve Atkiss, a former CBP chief of staff and now a partner at Command Consulting Group. “It’s much more difficult and expensive now.”

Madai Ledezma crossed the Mexican border into Texas a decade ago at age 23 and remains in the United States as an illegal immigrant. She said her uncle and brother had recently wanted to join her. But, she said, they’re staying put after her uncle was caught by the Border Patrol a year ago and locked up for a month before being sent back to Mexico.

“The risk of crossing again is that he will be locked up again,’’ Ledezma said. She added, “I just heard recently that the Border Patrol now has the ability to fire their weapons.”

‘Aging in place’

Ledezma’s uncle was one of a shrinking number of undocumented immigrants stopped by the Border Patrol. Government officials widely cite that trend as evidence that the overall flow is also down.

In 2000, considered the peak of the flood of illegal Mexican migration, more than 1.6 million people were apprehended, according to DHS data. Those numbers have plunged to around 400,000 per year since 2012 and are down 28 percent in the first part of fiscal 2015 compared with last year. Even last year’s widely publicized spike in unaccompanied minors crossing the border from El Salvador, Guatemala and Honduras has receded dramatically, the data show.

While the declining number of apprehensions is not conclusive proof that illegal immigration is down, other less publicized research strongly suggests this is the case.

Wayne Cornelius, director of the Mexican migration field research program at the University of California at San Diego, interviews hundreds of people each year in the Mexican state of Yuca­tan and asks them whether they are planning to come to the United States in the next 12 months.

In 2006, 24 percent said yes. By 2009, as the U.S. economy was cratering, 8 percent said yes. This year, 2.5 percent answered in the affirmative.

A recent study by the Pew Research Center, meanwhile, found that the median length of stay for illegal immigrants in the United States jumped from less than eight years in 2003 to nearly 13 years by 2013. Their median age has increased from 28 during the 1990s to nearly 36 today.

Those figures wouldn’t be possible if young men were still coming across the border in huge numbers, and it was those young men who accounted for most of the illegal traffic.

But Massey, the Princeton researcher, highlighted an unintended consequence of the security crackdown on the border. He said immigrants who are already in the United States are afraid to go back and forth to Mexico as they traditionally did, and are “aging in place” in the United States.

Ledezma’s tale is a common one. Over the past decade, she and her husband, Jose Pina, a landscaper, have become involved in their community in New Carrollton. Their daughter, Heather, 6, is a U.S. citizen. Ledezma volunteers at Heather’s school, reads with her at the public library and attends a local church.

“After so many years of living here, I of course consider this my home,” she said.

According to estimates by the Migration Policy Institute, about a third of illegal immigrants own a home and have children who are U.S. citizens.

“We have this population here and they haven’t left and they don’t appear to be going back and forth to Mexico anymore,’’ said George Escobar, senior director of human services for CASA, a ­Maryland-based immigrant advocacy group. “These trends have reshaped the immigration debate right before our eyes.”